A control device, for an electric vehicle that runs by receiving AC power from an overhead line, generally is equipped with a main transformer for changing an AC voltage received from the overhead line. In many cases, the main transformer has secondary windings, each connected to a converter of a plurality of converters, and has a tertiary winding connected to a supplementary power supply. A pulse width modulation (PWM) converter is widely used in the converter connected to the secondary winding.
By operation of switching elements constituting circuitry of the PWM converter, a harmonic can be generated in a primary current flowing through a primary winding of the transformer. For example, Patent Literature 1 teaches a method for control of a PWM converter in order to cause a decrease of a harmonic component of the primary current.
In the method for control of the PWM converter described in Patent Literature 1, a modulation wave used for pulse width modulation control in a plurality of PWM converters is set to the same phase as that of the power supply voltage. Also, this method of control sets a predetermined phase difference between groups of PWM converters allocated among driven cars constituting a train, for a carrier wave used for pulse width modulation control of the PWM converters.
Further, generally strongly coupled windings among the secondary windings and the tertiary winding are affected by mutual inductance. Each of the strongly coupled windings is affected by mutual inductance if the operating state is non-uniform for the loads supplied with power, and thus the phase differences of the currents flowing in each of the windings may deviate from the predetermined values. Thus the method of control of PWM converters described in Patent Literature 1 possibly does not sufficiently decrease the harmonic component of the primary current.
Weak coupling between each of the secondary windings and the tertiary winding in order to decrease the effect of this type of mutual inductance is effective for making each of the mutual inductances nearly zero. Thus there are cases in which a separator and the like is arranged between each of the secondary and tertiary windings.